It has been found that in a 30 mph frontal barrier crash a restraining device such as an air bag must begin restraining the driver by the time the driver has moved forward about 5 inches if it is to prevent injury. Air bag inflating mechanisms require a period of time, such as 30 milliseconds, to effect inflation of the air bag once a signal initiating inflation has been received from a sensor.
It is also been determined that a passenger protective device is needed in accidents in which movement of the vehicle occupant relative to the vehicle reaches a velocity of 12 mph or greater. Thus, a sensor is required which will determine that the occupant will collide with some portion of the vehicle's interior at a speed of 12 mph or greater and makes this determination of a 30 mph frontal barrier or equivalent crash to initiate air bag inflation 30 a period of time such as milliseconds before the occupant has moved forward 5 inches.
It is important that the air bag inflation means be responsive to the accleration of the passenger compartment rather than some other part of the vehicle because this location leads to certain advantages compared with other sensor locations. For example, if a front fender or bumper of the vehicle should collide with a pole or the like which breaks upon or shortly after impact, a sensor mounted on the front fender or bumper could experience a velocity change of 12 mph or greater before the pole breaks, whereas the passenger compartment of the vehicle might experience a negligible velocity change. Under these circumstances, inflation of the air bag is not required and if inflation occurs, it might even contribute to a subsequent accident. It is important, therefore, that the sensor or sensors with which a vehicle is equipped be located in such positions and be of such construction as to be predictive that the passenger compartment will undergo a velocity change necessitating passenger protection.
The mechanical crash sensor designs (as distinct from electronic) that have been made in the largest quantities are the spring mass sensors. The term "spring mass" or "undamped spring mass" as used herein means those sensors wherein the motion of the sensing mass is controlled only by the acceleration experienced by the sensor, its translational or rotational inertia and a bias force such as would result from a spring, a magnet or a rolomite type band. The spring mass sensor is known to function adequately in the case of crash pulses of extremely short durations. Such pulses are characteristic of head on crashes or standard barrier impacts. When the vehicle on which the sensor is mounted experience a deceleration level greater than the biasing force exerted by the spring on the sensing means, such as would accompany a crash, the sensing mass moves rapidly towards the forward end of the vehicle and initiates air bag inflation.
Heretofore, all known mechanical sensors for sensing automotive crashes outside of the crush zone have essentially been acceleration level detectors. General Motors Corporation used a sensor mounted on the cowl in the 10,000 air bag cars built in the 1970's that was a magnetically biased undamped ball. The initial bias was approximately 20 g but fell off very rapidly as the ball traveled from its rest position. Thus, for all practical purposes, once an acceleration of the 20 g was experienced, the sensor would fire. Control Laser Corporation employed a sensor using two detent balls that provided initial bias of something over 7 g. Once that level of acceleration was experienced, very little additional energy was required to fire the system. Another example is disclosed in the Bell Pat. No. 4,116,132. In this patent the sensor is described to be "responsive to a predetermined level of acceleration force."
It has been determined that for a sensor located outside of the crush zone to satisfy the requirements stated above, the sensor must be designed so that the velocity change needed to fire the sensor must be significantly larger than the velocity change which has occurred when the sensor bias is overcome.
It has also been determined that a sensor mounted outside of the crush zone and used alone for sensing automobile crashes must be biased below 7 g. If such a sensor is biased below 7 g, it must not be an acceleration level detector, since a 7g level can occur in cases where an air bag is not wanted. These facts have not been appreciated heretofore by those designing mechanical sensors, and thus up until now, mechanical sensors have been level detectors.
The prior art mechanical sensors when located within the passenger compartment inevitably failed to distinguish between actual crash conditions requiring an air bag and those conditions not requiring an air bag. For example, conditions not requiring an air bag car bottoming, travel over a curb or railroad track or even a pot hole. Yet the prior art mechanical sensors could trigger under such circumstances to inflate an air bag unnecessarily.